1. Field of the Invention
The present disclosure relates to a method used in a communication device in a wireless communication system, and more particularly, to a method of cell search.
2. Description of the Prior Art
In a wireless communication system, an user equipment (UE) first performs cell search to obtain physical layer cell identity when the UE tries to access a network of the wireless communication system, and then performs synchronization with the cell. The UE should obtain the physical layer cell identity in the cell search as soon as possible, to be synchronized with the cell, so as to reduce time and frequency error.
In long term evolution (LTE) system, 504 physical layer cell identities (denoted as NIDcell) are allowed, and are divided into unique 168 physical layer cell identity groups (denoted as NID(1)), where each group consist of 3 physical layer identities (denoted as NID(2)). NID(1) is in the range of 0 to 167, NID(2) is in the range of 0 to 2, and NIDcell is expressed by NIDcell=3NID(1)+NID(2). In addition, the physical layer cell identity is carried by a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). As such, in order to get physical layer cell identity, the UE needs to detect the synchronization signals.
LTE system has frequency division duplexing (FDD) and time division duplexing (TDD) standards. PSS and SSS are transmitted twice per radio frame, namely, a period of 5 ms. In other words, the UE in cell search detects synchronization signals (i.e., PSS detection and SSS detection) with a period of 5 ms, and uses the correlations of PSS and SSS, to get downlink time domain and frequency domain synchronization.
In detail, PSS is constructed from a Zadoff-Chu sequence of length 63 and mapped into the first 31 subcarriers which are spaced on either side of the DC subcarrier. PSS sequence du(n) is expressed by:
            d      u        ⁡          (      n      )        =      {                                        e                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      un                    ⁡                                          (                                              n                        +                        1                                            )                                                                      63                                                                                        n              =              0                        ,            1            ,            …            ⁢                                                  ,            30                                                            e                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      u                    ⁡                                          (                                              n                        +                        1                                            )                                                        ⁢                                      (                                          n                      +                      2                                        )                                                  63                                                                                        n              =              31                        ,            32            ,            …            ⁢                                                  ,            61                              In a word, PSS sequence du(n) is generated according to a root index u. Reference is made to FIG. 1, which illustrates a correspondence between physical layer identity NID(2) and root index u. As shown in FIG. 1, physical layer identities NID(2)=0, 1, 2 respectively correspond to root indexes u=25 , 29, 34 of Zadoff-Chu sequence. As such, PSS sequence du(n) includes three sequences with different root indexes.
On the other hand, SSS sequence is constructed by two binary sequences, each of length 31, and mapped into the first 31 subcarriers which are spaced on either side of the DC subcarrier. These two sequences are scrambled by a scramble sequence, which is related to physical layer identity NID(2). Therefore, the UE needs to detect PSS first to obtain physical layer identity NID(2), and then utilizes physical layer identity NID(2) to generate the scramble sequence for SSS detection. SSS is transmitted in subframe 0 or subframe 5, and is expressed by:
      d    ⁡          (              2        ⁢                                  ⁢        n            )        =      {                                                                                                            s                    0                                          (                                              m                        0                                            )                                                        ⁡                                      (                    n                    )                                                  ⁢                                                      c                    0                                    ⁡                                      (                    n                    )                                                                                                      in                ⁢                                                                  ⁢                subframe                ⁢                                                                  ⁢                0                                                                                                                              s                    1                                          (                                              m                        1                                            )                                                        ⁡                                      (                    n                    )                                                  ⁢                                                      c                    0                                    ⁡                                      (                    n                    )                                                                                                      in                ⁢                                                                  ⁢                subframe                ⁢                                                                  ⁢                5                                                    ⁢                                  ⁢                  d          ⁡                      (                                          2                ⁢                                                                  ⁢                n                            +              1                        )                              =              {                                                                                                  s                    1                                          (                                              m                        1                                            )                                                        ⁡                                      (                    n                    )                                                  ⁢                                                      c                    1                                    ⁡                                      (                    n                    )                                                  ⁢                                                      z                    1                                          (                                              m                        0                                            )                                                        ⁡                                      (                    n                    )                                                                                                      in                ⁢                                                                  ⁢                subframe                ⁢                                                                  ⁢                0                                                                                                                              s                    0                                          (                                              m                        0                                            )                                                        ⁡                                      (                    n                    )                                                  ⁢                                                      c                    1                                    ⁡                                      (                    n                    )                                                  ⁢                                                      z                    1                                          (                                              m                        1                                            )                                                        ⁡                                      (                    n                    )                                                                                                      in                ⁢                                                                  ⁢                subframe                ⁢                                                                  ⁢                5                                                        where 0≤n≤30, parameter m represents shift index, in which parameters m0 and m1 are derived from the parameter m, and vectors c, s and z represent m-sequences. Detailed description for these parameters and vectors is as following: shift indexes m0 and m1 obtained by physical layer cell identity group NID(1) are expressed as:
            m      0        =                  m        ′            ⁢      mod      ⁢                          ⁢      31                  m      1        =                  (                              m            0                    +                      ⌊                                          m                ′                            /              31                        ⌋                    +          1                )            ⁢      mod      ⁢                          ⁢      31                          m        ′            =                        N          ID                      (            1            )                          +                              q            ⁡                          (                              q                +                1                            )                                /          2                      ,                  ⁢          q      =              ⌊                                            N              ID                              (                1                )                                      +                                                            q                  ′                                ⁡                                  (                                                            q                      ′                                        +                    1                                    )                                            /              2                                30                ⌋              ,                  ⁢                  q        ′            =              ⌊                              N            ID                          (              1              )                                /          30                ⌋            Sequences s0(m0) and s1(m1) are obtained by m-sequence {tilde over (s)}(n) with different cyclic shift of m0 and m1, and is expressed as:s0(m0)(n)={tilde over (s)}((n+m0)mod31)s1(m1)(n)={tilde over (s)}((n+m1)mod31)Scramble sequence c0(n) and c1(n) are related to NID(2), and obtained by m-sequence {tilde over (c)}(n) with the following equation:c0(n)={tilde over (c)}((n+NID(2))mod31)c1(n)={tilde over (c)}((n+NID(2)+3)mod31)Sequences z1(m0) and z1(m1) are obtained by m-sequence {tilde over (z)}(n) with the following equation:z1(m0)(n)={tilde over (z)}((n+(m0mod8))mod31)z1(m1)(n)={tilde over (z)}((n+(m1mod8))mod31)Reference is made to FIG. 2. In the cell search, the UE detects PSS and then detects SSS based on the detected PSS type (i.e., three root indexes u). PSS and SSS are both deployed within bandwidth of 1.4 MHz, and detected at sample rate of 1.92 MHz. The UE first applies decimation filter to decimate received signals with different sampling frequencies (e.g. 30.72 MHz, 23.04 MHz, . . . , 1.92 MHz) to frequency of 1.92 MHz, and generates three 128-point PSS in the time domain according to three root indexes (i.e., NID(2)=0,1,2). Then, the UE performs correlation on the 128 points PSS in the time domain with the decimated signal. PSS correlation result is outputted to a non-coherent-combining buffer for combining every PSS correlation results in the half frame (5 ms), so as to reduce interference of background noise.Non-coherent-combining buffer requires size for storing at least 28.8 k samples for one PSS (i.e., 9600 samples detected in half-frame in the time domain, and there are 3 PSS types. Therefore, there are 9600×3=28.8 k samples in the time domain). Moreover, peak search is used for finding the maximum value in the non-coherent-combining buffer at each PSS reception time. Then, the UE compares the maximum value with a threshold for determining whether a PSS is detected, and performs SSS detection with NID(2) corresponding to the maximum value and with the PSS reception time if the maximum value is larger than the threshold. In other words, if the maximum value is smaller than the threshold, the UE determines that the PSS is not detected, and therefore does not perform SSS detection. SSS detection is applied with coherent or non-coherent method, which is used for transforming the received signal in the time domain to the frequency domain by Fast Fourier transform (FFT) with PSS reception time obtained by the peak search and NID(2), and performing correlation on each of SSS sequences corresponding to 168 physical layer cell identity group NID(1) with the transformed signal. In addition, the UE selects the maximum value from the 168 correlation results, and therefore finds the corresponding physical layer cell identity group NID(1). Finally, the UE can obtain physical layer cell identity NIDcell based on the NID(2) and physical layer cell identity group NID(1).
Conventional cell search is not strictly required for frame synchronization, and thus conventional cell search can be used in FDD-LTE mode system. However, for TDD-LTE mode system, frame synchronization is severely important . Thus, conventional cell search is not fit in TDD-LTE mode system with time requirement. For example, small cell is required of frame synchronization less than 3 μs. For OFDM symbol length of 66.67 μs, synchronization signals (i.e., PSS and SSS) transmitted from different cells are overlapping at receiver (i.e., UE), which causes interference between synchronization signals. In other words, strong synchronization signals will affect the detection probability of weak synchronization signals.